1. Field
The present disclosure relates to radio frequency (RF) signal interface circuits, and particularly to a complex-pole load used to provide channel selection and image rejection. Such complex pole loads are useful in baluns, mixers and filters.
2. Background
Ultra-low-power (ULP) radios have essentially underpinned the development of short-range wireless technologies such as personal/body area networks and Internet of Things. The main challenges faced by those ULP radios are the stringent power and area budgets, and the pressure of minimum external components to save cost and system volume. Balancing them with the performance metrics such as noise figure (NF), linearity and input matching involves many design tradeoffs at both architecture and circuit levels.
Ultra-low-voltage receivers have been extensively studied for short-range ZigBee, Bluetooth and energy-harvesting applications. Yet, the lack of voltage headroom will limit the signal swing and transistor's fT, imposing the need of bulky inductors or transformers to facilitate the biasing and tune out the parasitics. Thus, the die area is easily penalized, such as 5.76 mm2 in and 2.5 mm2 in one example. The current-reuse topologies should benefit more from technology scaling when the NF is less demanding. Advanced process nodes such as 65 nm CMOS feature sufficiently high-fT and low-VT transistors for GHz circuits to operate at very small bias currents. Unsurprisingly, when cascading the building blocks for current reuse, such as the low-noise amplifier (LNA) plus mixer, the RF bandwidth and linearity can be improved as well, by avoiding any high-impedance nodes at their interface.
Several NF relaxed current-reuse receivers have been reported. An example of an LNA mixer voltage controlled oscillator (VCO) (LMV) cell is illustrated in FIG. 1. In this example, the mixer uses an external LMV cell Lext for narrowband input matching and pre-gain. One LC-tank VCO saves the chip area, but putting the I/Q generation in the LNA (M1-M2) degrades the NF. Only single-balanced mixers (M3-M4) can be used.
Sharing the bias current among more blocks saves power (2.4 mW), but the NF, gain and the input port voltage reflection coefficient (S11) are sensitive to its external high-Q inductor (Lext) for narrowband input matching and passive pre-gain. Also, under the same bias current, it is hard to optimize the LNA's NF (RF path) with the phase noise of the VCO (LO path). Finally, although a single VCO can save area, the I/Q generation has to be embedded into the LNA. This structure leads to a 3 dB gain loss deteriorating the NF (12 dB), while rendering the I/Q accuracy more susceptible to process variations.
To return the I/Q generation back to the LO path, one GPS receiver design adopts two VCOs to tailor a quadrature LMV (QLMV) cell. Although its power is further optimized (1 mW), three on-chip inductors and one off-chip balun are entailed, penalizing die size and system costs. Also, both LMV and QLMV cells share the same pitfall that only a 50% duty cycle LO (50% LO) can be used for the mixing, which is less effective than 25% LO in terms of gain (i.e., 3 dB higher), NF and I/Q isolation. Finally, as their baseband (BB) channel selection and image rejection are out of their current-reuse paths, any large out-band blockers are necessarily converted into voltages before filtering. This fact constitutes a hard tradeoff between noise, linearity and power (i.e., 1.2 mW baseband power in one example and 5.2 mW baseband power in the above-mentioned GPS receiver).
Another example is a current-reuse circuit-reuse receiver, which merges the RF LNA and baseband transimpedance amplifier (TIA) in one cell, shown in FIG. 2A, in which a circuit-reuse receiver merges an RF LNA and BB TIA. A conceptual view of its operation is given in FIG. 2B, which shows a single-ended equivalent circuit of FIG. 2A, illustrating its RF-to-BB operation conceptually (from right to left).
Without the VCO, and by using passive mixers, this topology can reserve more voltage headroom for the dynamic range. An RF balun is nevertheless entailed for its fully-differential operation, and several constraints limit its NF and linearity: 1) the LNA and TIA must be biased at the same current; 2) the LNA's NF should benefit more from short-channel devices for M1-2, but the baseband TIA prefers long-channel ones to lower the 1/f noise; and 3) any out-band blockers will be amplified at the LNA's (TIA's) output before deep baseband filtering.